It is difficult under certain circumstance to measure various voltages of a device under test (DUT), particularly when the DUT includes one or more field-effect transistors (FETs). There are a number of conventional methods used for measuring the threshold voltage of a FET. For example, according to a conventional linear search method, a gate voltage of the FET is swept between lower and upper limits of a predetermined search range, while drain current or source current flowing through the FET is measured for each gate voltage. When the gate voltage causes the drain current or the source current to obtain the desired value, the measurement is finished. However, the overall measurement time of the linear search method may be rather long because the measurement of the drain current or the source current at each gate voltage requires a sufficient convergence waiting time, which must be repeated several times at close intervals in order to obtain necessary resolution.
Another example is a conventional binary search method, according to which the gate voltage of the FET is swept between lower and upper limits of a predetermined search range, again, while the drain current or the source current flowing through the FET is measured for each gate voltage. When the gate voltage causes the drain current or the source current to obtain the desired value, the measurement is finished. By determining a setting of the gate voltage based on the binary search method, the number of times that the gate voltage is set until the search is finished is generally less than using the linear search method, thus shortening measurement time. In this method, on the assumption that there are N points of measurement from the lower limit to the upper limit of the search range based on resolution, the search may be completed with log 2N times of measurement at most, and the gate voltage is therefore applied less often than in the linear search method.
However, in order to perform the binary search method measurement with high precision, the gate voltage is set and applied a sufficient number of times to obtain the desired high resolution. The operation of performing the measurement by waiting for the corresponding convergence is repeated, and hence the binary search method is disadvantageous in that the overall measurement time becomes longer. In addition, a voltage higher than a convergence value of the voltage may be applied as the gate voltage before the convergence value is finally determined. Therefore, due to influences of thermal stress on the particular DUT, the binary search method may actually be inferior to the linear search method with respect to precision, reproducibility, and stability of the measured value. For example, the measured values may fluctuate due to stress on the DUT.
Thus, generally, conventional measurement devices and corresponding methods have a number of drawbacks. For example, setting the gate voltage and measuring the current are typically performed to search for the threshold voltage, and hence the measurement requires much time. Also, feedback gain of the measurement device may be difficult to set, resulting in taking too wide of a margin, which increases convergence time. In addition, only a source current can be fed back.